Organic disulfides are useful as chemicals for pre-sulfiding catalysts and as chemical intermediates in the production of agricultural and pharmaceutical products. Organic disulfides are produced by oxidation of mercaptans according to the general reaction: EQU 2R-SH+Oxidant.fwdarw.R-S-S-R+Reductant, (1)
wherein R is a hydrocarby radical.
The most common oxidants for consideration are sulfur, hydrogen peroxide, a reducible metal ion, oxygen and sulfur dioxide.
Examples of these are as follows:
2 R-SH + S .fwdarw. R-S-S-R + H.sub.2 S (2) 2 R-SH + H.sub.2 O.sub.2 .fwdarw. R-S-S-R + 2 H.sub.2 O (3) 2 R-SH + 2 Fe.sup.+3 .fwdarw. R-S-S-R + 2 Fe.sup.+2 + 2H.sup.+ (4) 4 R-SH + O.sub.2 .fwdarw. 2R-S-S-R + 2H.sub.2 O (5) 4 R-SH + SO.sub.2 .fwdarw. 2 R-S-S-R + 2 H.sub.2 O + S (6a) 2 R-SH + S .fwdarw. R-S-S-R + H.sub.2 S (6b) 6 R-SH + SO.sub.2 .fwdarw. 3 R-S-S-R + 2 H.sub.2 O + H.sub.2 S (6a + 6b)
In order to minimize production of polysulfides, when molten sulfur is used as the oxidant, excess mercaptan is used, generally in at least 50-100% excess. The mercaptan then needs to be recovered from the by-product hydrogen sulfide for recycle. Also, the organic disulfide must be recovered from the polysulfides formed.
The use of hydrogen peroxide suffers from the production of 2 moles of water per mole of organic disulfide produced as well as the additional water present with the aqueous solution of hydrogen peroxide. Thus, the reactor volume productivity is low for this method.
The use of a stoichiometric amount of metal ion such as ferric ion (Fe.sup.+3) is a possibility (reaction 4). However, this process also suffers from low volume productivity of organic disulfide produced per reactor volume. The advantage is that the ferrous ion (Fe.sup.+2) can be regenerated with air to produce ferric ion (Fe.sup.+3): EQU 2 Fe.sup.+2 +O.sub.2 +2H.sup.+.fwdarw.2Fe.sup.+3 +H.sub.2 O.
The use of oxygen as the oxidant (reaction 5) can achieve high conversions (&gt;99.5%) with high organic disulfide selectivity (&gt;98%). The reactions are done in the presence of a basic liquid oxidation catalyst, such as caustic, or triethylamine.
The use of oxygen does have a significant potential safety issue due to the potential for explosions under certain conditions. For instance, at concentrations higher than 10 volume % oxygen, there is a risk of explosion if there is an ignition source present.
The other alternative is to use sulfur dioxide as the oxidant (reaction 6a+6b) in the presence of a liquid catalyst. The reaction is typically carried out in the presence of a basic liquid oxidation catalyst, such as triethylamine (C.sub.2 H.sub.5).sub.3 N or boron trifluoride etherate (C.sub.2 H.sub.5).sub.2 O.circle-solid.BF.sub.3. The use of sulfur dioxide as the oxidant offers the advantage of less explosion hazard as compared to the use of oxygen, and decreased handling difficulties compared to the use of molten sulfur. On the other hand, the use of a liquid basic catalyst in thin method has the disadvantage of lower reactor throughput, lower product yield and lower product selectivity as compared to the use of a solid catalyst in a continuous flow reactor. Also, there is the need to separate the liquid basic catalyst from the liquid product and unreacted liquid feedstock.
Therefore, there is a need to develop a process for oxidizing a mercaptan to an organic disulfide using sulfur dioxide in the presence of a solid catalyst, and, optionally, in the presence of an oxygenated hydrocarbon, and achieving the highest possible yield of organic disulfide.